What is the default transaction isolation level for SQL Server with ADO.NET?

READ COMMITTED is the default isolation level for the Microsoft SQL Server Database Engine.

Source:

• Customizing Transaction Isolation Level @ MSDN.

Here is how it compares to other isolation levels:

• Transaction Isolation Levels @ MSDN.

The MSDN documentation for SqlConnection.BeginTransaction() also states Read committed

... To reset the isolation level to the default (READ COMMITTED) ...

• MSDN article

The accepted answer by hkf gives the correct answer for transactions started manually with SqlConnection.BeginTransaction(). Here, the default level is ReadCommitted.

However, this is not the only way to start a new transaction in ADO.NET: Transactions can also be created automatically by using the classes from the System.Transactions namespace, in particular, by creating a TransactionScope.

Contrary to transactions started manually, transactions created by the System.Transactions infrastructure (and, thus, by a TransactionScope) are Serializable.

See below link for more information:

• IsolationLevel Enumeration @ MSDN.